A team of Iranian spelunkers exploring an underground cave system near Mehriz, Yazd have made a surprising discovery. Last year, the group of cavers stumbled across a massive underground chamber and after taking careful and exact measurements of its size, the cave is now confirmed to be the fourth largest in the world.

This new cavern is part of the Ghar-e-Dosar cave system located in central Iran, which has been known about for some time and is a popular destination for cavers in the region. The system hasn’t been completely mapped however so new aspects of it are still being discovered. Last February a team of eight entered the largest chamber there and set about taking precise measurements of its total expanse. Those numbers have all been crunched now and what they found is simply amazing.

The subterranean room measures 1263 feet across and 869 feet wide, covering roughly the equivalent of 20 acres. The cavern slopes downward over its length starting with a height of about 230 feet and plunging as low as 469 feet at its deepest point. Those dimensions are large enough to put it above Majlis al Jinn cavern in Oman and slightly behind the Torca del Carlista in Spain on the list of the world’s largest cave rooms.

It should be noted that this chamber ranks as the fourth largest in terms of size for a single cavern. Ghar-e-Dosar isn’t nearly as large as Mammoth Cave National Park in Kentucky, however, which doesn’t have a single chamber that compares in size but does stretch for more than 400 miles of underground passageways.